The vast majority of industrial processes, by definition, consist of a series of sequential or concurrent steps, each step involving one or more actions to be taken by a machine or machines. The steps may occur at specific times and in a specified sequence according to specific parameters, or may occur in response to specific events. Each step may have one or more elements, each element describing activities or operations with greater specificity.
In the past, industrial equipment was commonly controlled directly by interfacing the equipment with a programmable logic controller, or "PLC". A PLC is a solid-state device designed to perform logic functions previously accomplished by electromechanical relays. The PLC uses output modules to actuate the industrial equipment in response to physical stimuli which the PLC is programmed by the operator of the system to recognize through input modules. PLCs, which still find wide use today, are usually programmed using either ladder logic or sequential function charts. Because of the cryptic nature of ladder logic, it is inherently complex and difficult and time consuming to debug and maintain.
More recently, manufacturers have sought to take advantage of the greater flexibility of general-purpose computers, including inexpensive commercially available personal computers, or "PCs", to enhance the efficiency associated with creating and maintaining software programs used to control industrial processes. Because general purpose computers can be programmed in high level commercially available languages such as BASIC, FORTRAN, C, or in object-oriented languages such as C++, manufacturers and process control vendors have been able to develop PC-based control systems that emulate traditional PLC functions, but do it in such a way that permits them to be easy to use, program and maintain, while still offering significant cost savings over dedicated PLC-based solutions.
In many instances when a PLC is used, the PLC is connected to a central control computer. In such an arrangement, the PLC plays its own dedicated role controlling the industrial process at hand while concurrently communicating information back to the central computer. By using the high level commercially available programming languages, control methods have evolved using graphical flow charts to aid the software programmer developing control programs which can emulate traditional PLC functions. Use of PCs in this way enable a manufacturer to develop and run the operator interface on the same PC and share data with other WINDOWS based programs through dynamic data exchange. Thus, a single PC may perform the function of the programmable logic controller, the operator panel, the programming terminal, and the real time system simulator. A PC therefore can replace three separate components: PLC programming terminal, PLC processor, and operator interface. By implementing a PC-based control system, manufacturers are able to lower control system investment costs, increase productivity in design and industrial operations, and reduce down time with built in flow chart based diagnostics.
The development of any application program requires that the developer understand how the program executes its logic steps. In this phase of control application development, often known as "debugging", the developer must have the ability to inspect variables and their values. Typically, this requires the user to analyze data and correlate that data with the control program, which can be extremely burdensome on the programmer. However, there also must be a way to detect where a program is executing because simple evaluation of data may limit the programmer's understanding of the program only to the microscopic effects rather than providing an understanding of the program's effects at the transaction level. One particular difficulty is that the controlling program must often execute many times per second whereas the application engineer is only capable of comprehending information at a much slower rate. Additionally, multiple variables may change their values several times during any cycle of the controlling program. Debugging is usually a time-consuming and frustrating process for the applications engineer. The frustration can be reduced by using object oriented programming techniques. Manufacturers and applications engineers are therefore very interested in new methods for increasing the efficiency associated with developing control programs.